(1) Field of the Invention
The present invention relates to an apparatus and a method for detecting abnormal conditions of a motor.
(2) Description of the Related Art
Conventional motors are used as drive sources for, for example, cooling fans and fuel pumps mounted on motor vehicles. Some devices for driving these motors have a switch device such as a field-effect transistor (FET) which is provided on a current path from a power source to the motor. This switch device allows stepless speed control of a rotational speed of the motor by changing a duty cycle of power supply voltage provided to the motor.
However, motor windings and switch devices of such conventional motors may be thermally destructed when a motor lock or a short circuit occurs and as a result, overcurrent continuously flows. To avoid such overcurrent caused by the motor lock or the short circuit, various methods for detecting abnormal conditions are in use. For example, the motor driving apparatuses as disclosed in the cited documents 1 (Japanese Unexamined Patent Application Publication No. 2001-298988) and 2 (Japanese Unexamined Patent Application Publication No. H10-8959) detect an occurrence of the abnormal condition when a current flowing through the motor exceeds a predetermined threshold value. Some of known methods for detecting abnormal conditions detect the abnormal conditions based upon a value of a voltage between a drain terminal and a source terminal of a field-effect transistor provided as the switch device, for this voltage corresponding to the value of the current flowing through the motor.
Methods for detecting abnormal conditions based upon the current flowing through the motor as mentioned above, however, fail to detect the abnormal condition in a case of an interlayer short circuit, where intermittent flow of overcurrent occurs during a short period of time. Since such momentary overcurrent as in the interlayer short circuit is hardly detected and therefore the interlayer short circuit is left undetectable as an abnormal condition. Also, since a value of the overcurrent in the interlayer short circuit is smaller than that in a case of a dead short circuit, the overcurrent caused by the interlayer short circuit in some cases may not exceed a predetermined threshold. Nevertheless, intermittent overcurrent resulting from the interlayer short circuit can cause gradual rise in a temperature of the switch device and, consequently, thermal destruction of the switch device may result.
In addition, the methods for detecting abnormal conditions based upon the current flowing through the motor as mentioned above fail to detect the abnormal conditions accurately when heat radiation efficiency of the switch device is degraded across the ages. This denotes that the heat radiation efficiency degraded across the ages could trigger the thermal destruction with the current flowing through the motor which would not otherwise cause the thermal destruction.